1. Field of the Invention
The present invention relates to a fixing apparatus of a thermal roller fixing type. More particularly, the invention relates to a fixing apparatus for fixing a toner image on a sheet, which comprises a fixing roller with a heater therein and a pressing roller urged thereagainst.
2. Description of the Related Background Art
Fixation is in general performed using a fixing roller and a pressing roller in a fixing apparatus of image forming apparatus such as electronic copier, printer, or facsimile. The fixing roller is formed by coating a core with a resin like polytetrafluoroethylene ( PTFE ) such as Teflon ( registered trademark ) and is provided with a heater inside the core. The pressing roller is urged against the fixing roller to form a nip portion therebetween. A transfer sheet carrying a toner image is passed through, the nip portion for fixation. A surface layer of the pressing roller is usually made of an elastic material such as silicone rubber suitable for formation of the nip portion.
A temperature is desirably even in a longitudinal direction of the fixing and pressing rollers. An uneven temperature could cause wrinkles on the transfer sheet, an unfixed portion of image, or offset due to a difference in nipping rising from a difference in thermal expansion between portions different in temperature.
The surface temperature of the fixing roller is detected by a temperature detecting member such as a thermistor in contact with the fixing roller. The heater is controlled to maintain the surface temperature of the fixing roller at a predetermined value based on the detection results. An output distribution of the heater is set non-uniform, because the imaging portion contacting with the record sheet requires a greater quantity of heat than the nonimaging portion not contacting with the record sheet, for thermal transfer to the record sheet.
When the surface temperature of the fixing roller is detected by a temperature detecting member in contact with the imaging portion in a conventional fixing apparatus, a release member thereof made for example of Teflon is abraded by the temperature detecting member to cause incomplete fixation of toner or wrinkles on the record sheet in the abraded part, degrading a quality of image. Further, thermal accumulation tends to take place in the non-imaging portion due to no heat transmission to the record sheet. This thermal accumulation could cause an abnormally high temperature in the non-imaging portion after continuous fixations though the imaging portion is maintained at a predetermined temperature. This results in deformation or melting of the pressing roller urged against the fixing roller.
In contrast, when the surface temperature of the fixing roller is detected by a temperature detecting member in contact with the non-imaging portion in the conventional fixing apparatus, an abnormally high temperature could happen in the imaging portion while keeping a predetermined temperature of the non-imaging portion, because the heater has a higher output in the imaging portion than in the non-imaging portion. This would, also result in deformation or melting of the pressing roller urged against the fixing roller. On the contrary, the temperature in the imaging portion will be reduced below a fixation possible temperature due to thermal transmission to the record sheets in the imaging portion after continuous fixations while maintaining the non-imaging portion at the predetermined temperature, causing incomplete fixation.
Further, thermal leakage takes place in the conventional fixing apparatus through portions in contact with the fixing roller having the heater, for example, bearings, drive gears, and so on, upon start of heating from a cool state of rollers. The thermal leakage lowers a thermal efficiency and a temperature at the ends of the fixing roller, causing incomplete fixation. To prevent the thermal leakage, a fixing apparatus has a thermal insulation bush between the fixing roller and a bearing disposed on a side plate, making the arrangement complicated.
Also, after the fixing apparatus is heated to a stable condition, continuous sheet passage of transfer sheets of small size causes a temperature difference between the sheet passage area and the non-passage area of the roller, resulting in temperature rise in the non-passage area of the roller ends. If a transfer sheet of larger size is passed immediately after the continuous sheet passage of smaller size sheets, there would be caused problems such as incomplete fixation due to the lowered temperature in the sheet passage area of smaller size sheets and offset due to the higher temperature in the non-passage area. The repetition of temperature rise in the roller ends would result in deterioration of fixing and pressing rollers, inevitably shortening their lives.